I will explain how to put a picture on the header/top or anywhere else (about an attachment, read Power Automate: add an attachment in an email). Before to start:

• Image can be in PNG or JPG
• Steps should be outside of a “control” or “condition” step

After selecting the kind of flow I need, I click on the “+” to insert a new step:

In the “search” field, put “sharepoint get file content” to select this option:

NOTE: my picture is in sharepoint, if you have it in other cloud like onedrive, use/select the corresponding one

Fill the fields:

Click on the “+” then in the “search”, put “initialize” to select this option:

Fill the fields:

• Name: put whatever I want, for my example “picturemail” (this name will be used for my email)
• Type: select “string”
• Value:
  • Put: <img src="data:image/jpeg;base64, "/> then click on the “space” then on “fx”
    
  • Put: .$content then click at the beginning just before the dot
    
  • Click on “dynamic content” then on “file content”. Once done, click on “add”
    
  • Result: <img src="data:image/jpeg;base64,@{body('Get_file_content_using_path').$content} "/>
  • NOTE:
    
    • To specify the size, add “width="XXX" height="XXX"” like that:
      
    • To put a link, add <a href="link">imagecode</a>:
      

Alternatively, I can use “compose” instead of “initialize variable”. I will put <img src=' '/> then in the space, click on “fx”:

Put dataUri() and between the parenthesis, add “file content”:

Result: <img src='dataUri(body('Get_file_content')) '/>

Click on the “+” then in the “search”, put “send email” to select this option:

Fill the fields and in the body, click on the “lightning” icon to select:

Once done, click on “save” and “test”:

If all go well, this is my email: